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Résumé séminaire A.J. Beaudoin

"Grain Boundary Delamination in the Fracture of Al-Li Alloys"

Aluminum-Lithium alloys are under continuing development, with increasing application in aerospace structures. In the fracture of these materials, it is common to develop delamination, a separation of grain boundaries perpendicular to the primary crack. There are two types of delaminations, depending on the orientation of the primary crack and the extended grain boundaries lying in the rolling plane: the propagation direction of the primary crack lies in the plane of the crack divider delamination, which effectively “divides” a mode I crack in plane strain into a number of mode I cracks in plane stress; the crack arrestor delamination acts to turn the primary crack, transforming a mode I crack into a mode II one. This present work is directed at establishing the mechanical underpinnings, acting at the mesoscale, that drive the nucleation and propagation of delaminations. Mechanical characterization is achieved through fracture toughness and fatigue studies. Digital Image Correlation is employed in mechanical testing to identify the onset of plastic localization, prior to the development of delamination. Electron Back Scatter Diffraction mapping methods are used in the characterization of sectioned samples. Collectively, this data is then applied to pose detailed models of a delamination in the presence of a primary crack. Both "crack divider" and "crack arrestor" delaminations are studied in the present work. The measurement of crystallographic orientation in the vicinity of the delamination is utilized in the crystal plasticity formulation. Results indicate the key role of slip incompatibility: a grain oriented favorably for slip sharing a boundary with another with "less favorable" crystallographic orientation promotes the development of a shear stress that sustains delamination. This work is supported by the NASA Marshall Flight Center through grant NNX09AN21G.

Armand J. Beaudoin
Dept. of Mechanical Science & Engineering
University of Illinois at Urbana-Champaign

 

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